A passive optical network (PON) is a flexible access network that is capable of providing a range of broadband and narrow-band services for business and residential customers. The underlying equipment is considered to be relatively inexpensive for network operators because they do not require any active equipment or power supplies between the operator's central office (CO) and customer's premises (CP). As shown in the PON 10 of FIG. 1, downstream PON traffic is destined from the Optical Line Termination (OLT) 12 residing in the CO towards a number of network terminals 16, termed Optical Network Termination (ONT) devices or Optical Network Units, residing on the CP via an optical splitter 14.
Most PON networks and all major PON standards use non-modulated, baseband Non-Return to Zero (NRZ) signaling over the optical fiber: Time Division Multiplexing (TDM) in the downstream direction and Time Division Multiple Access (TDMA) in the upstream. So far, such signaling has been proven as the most cost-effective choice.
The increase in the TDM/TDMA PON bandwidth is achieved simply by speeding up the downstream and upstream baseband signaling. The evolution of the PON systems and standards has seen a steady increase of PON bit rate ranging from the initial 155 Mb/s in APON in the mid-1990s, to 1.25 Gb/s in Gigabit-capable PON (GPON) [ITU-T G.984] and Ethernet PON (EPON) [IEEE 802.3ah] of mid-2000s, up to 10 Gb/s specified in the IEEE 802.3av (10GEPON) [1-3] and ITU-T 10G GPON standards as of year 2009. Whereas this trend has been delivering ever-higher bandwidth to the subscribers, it has also been increasing the ONU energy consumption proportionally to the bit rate.
ONU energy consumption is important for two reasons. First, given that key communication services such as residential phone lines and internet access depend on the ONU, it is necessary to ensure its long operation when powered by a battery during utility power outages. To that end, it is desirable to minimize the operational energy consumption of the ONU. Secondly, ONU energy efficiency (and its carbon footprint) is becoming increasingly important in the context of environmental concerns and rational use of energy resources.
Patterns of ONU energy consumption in processing upstream and downstream traffic are notably different. The upstream activity is bursty and confined in time-slots in which the ONU transmits upstream data, whereas its downstream activity is continuous. The energy consumption of ONU upstream and downstream processing units, if normalized per unit of subscriber traffic, turns out to be considerably higher for the downstream traffic than for the upstream. This fact is attributable to the nature of GPON and EPON protocols, which requires the ONU physical (PHY) and most of the media access control (MAC) layer hardware to be turned on and continuously receive downstream traffic. Downstream payload not destined to the subscriber associated with the particular ONU is identified and dropped by the ONU, but only after it causes significant amount of energy dissipation.
Proposed solutions for ONU low-power operation have been limited to reduction of ONU functionality during battery powered operation and scheduled inactivity (sleep) intervals.
What is required is a system and method for reducing the ONU energy (and power) consumption during its normal operation.